1. Field of the Invention
The present invention relates to an apparatus for drying under reduced pressure which dries a substrate having a coating liquid such as a resist applied on its surface in a pressure-reduced atmosphere, a coating film forming apparatus employing the apparatus for reducing pressure, and a method of drying under reduced pressure.
2. Description of the Background Art
A spin coating method with which a resist liquid is applied while a substrate is rotated is commonly used for forming a resist film used in photolithography. Recently, a method of applying the resist liquid with a nozzle in such a manner as one stroke writing has been developed. As a resist liquid used in such an applying method, a solvent with low volatility, such as high-boiling thinner, is usually employed as a solvent dissolving a resist component. In this case, in order to dry the resist liquid in a short time after it is applied, drying under reduced pressure is advantageous.
To that end, the applicant considers a unit for drying under reduced pressure shown in FIG. 10, for example. The unit for drying under reduced pressure includes a sealed container 1 constituted of a cover 10 and a mount portion 11. In a ceiling of cover 10, an exhaust hole 12 is provided. Exhaust hole 12 is connected to a vacuum pump 14 with a pipe 15 through a pressure regulating valve which is a pressure regulating portion, whereby pressure reduction down to a prescribed pressure is attained in sealed container 1. In addition, a straightening vane 16 capable of up and down movement is provided so that a component evaporated from the coating liquid forms a uniform exhaust stream on the surface of a wafer W.
In such a unit for drying under reduced pressure, wafer W having the resist liquid applied is mounted on mount portion 11, and straightening vane 16 is set to a position at a prescribed height so as to face wafer W. Then, a temperature of wafer W is adjusted by not-shown temperature adjustment means provided in mount portion 11, and vacuum pump 14 is actuated for pressure reduction and evacuation. Here, as shown in a section A of a pressure curve in FIG. 11, the pressure in sealed container 1 rapidly lowers. Thereafter, when the pressure reaches as low as the vapor pressure of the solvent, the solvent starts to evaporate, and the pressure gently lowers around the vapor pressure, as shown with a section B. When most of the solvent evaporates, the pressure again rapidly lowers, as shown in a section C. When the prescribed pressure is attained, vacuum pump 14 is stopped, and a treatment of drying under reduced pressure is completed. The pressure curve exhibits considerable variation, depending on a type of the solvent, an amount of the applied resist liquid, concentration of the resist component in the resist liquid, a temperature in drying, or a flow rate of the exhaust.
Meanwhile, a coating liquid film R on the surface of wafer W before drying exhibits a rounded corner shape due to surface tension of the coating liquid itself in a peripheral region of wafer W (an area extending inward from the periphery by approximately 20 mm, for example), as shown in FIG. 12A, for example. Accordingly, straightening vane 16 for correcting the rounding is provided so as to face the surface of wafer W. In other words, straightening vane 16 is positioned such that the solvent component that evaporates from coating liquid film R flows outward through a gap between straightening vane 16 and the surface of wafer W. When a current spreads coating liquid film R in an outward direction, the liquid is moved toward the outer periphery, thereby attaining evenness.
On the other hand, in the method of drying under reduced pressure as described above, an important parameter for correction is a time period for drying (time period of section B). When the exhaust flow rate during pressure reduction and evacuation is increased, the solvent actively evaporates, and the time period of section B is shortened. On the other hand, the solvent evaporates before the shape of coating liquid film R is corrected. In other words, drying is completed with the rounded shape in the peripheral portion still remaining, as shown in FIG. 12B. As a result, in an area extending inward from the periphery by approximately 30 mm, the film thickness decreases toward the outward direction, and the film exhibits such a shape that it sharply rises in a portion considerably close to the periphery. In contrast, if the exhaust flow rate is set smaller and the time period of section B is extended, a point attaining high in-plane uniformity in terms of thickness of the coating liquid film is overreached, and an area approximately 5 mm inside the periphery may be raised as shown in FIG. 12C, resulting in lower throughput.
With regard to pressure control within sealed container 1, the pressure change in section B is nominal, for example, 26.6 Pa (0.2 Torr). Therefore, it is difficult to control the pressure within sealed container 1 by operating valve lift of pressure regulating valve 13. Consequently, pressure regulating valve 13 is in a full open state while the solvent is actively evaporating. Therefore, the exhaust flow rate is determined by capability of vacuum pump 14, and adjustment of the drying time period (adjustment of the time period of section B) is achieved by selecting a type of the solvent. As a result, it has been difficult to adjust (condition) a parameter for drying under reduced pressure, and to obtain a coating film with high in-plane uniformity in terms of film thickness.